Latex composition and process of making



Patented Nov. 17, 1931 PATENT OFFICE WALLACE B. VAN ARSDEI: AND ROGER B. HILL, BERLIN, NEW HAMPSHIRE, AS-

SIGNORS TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE LATEX COMPOSITION AND PROCESS OF MAKING No Drawing.

Aqueous rubber dispersions, such as latex in stabilized or preserved condition, find diverse commercial applications, for instance, for the coating or impregnation of bod1e s,

l e. g., fibrous sheet material. The rubber s coagulated in a Continuous, coherent condition when the dispersion-treated body is dried, and imparts such properties as waterproofness and strength to the body. It is sometimes desired that the rubber be present in a vulcanized condition in the body, as vulcanized rubber is free from tackiness and has much higher tensile strength than unvulcanized rubber. The rubber particles of an aqueous rubber dispersion, such as latex,

may be vulcanized by the so-called Schidrowitz process, which involves adding vulcanlzing agents such as sulphur, preferably along with accelerators of vulcanization, to the dispersion, and then heating the dispersion to effect vulcanization of the rubber particles but without causing coagulation (see U. S.

Patent No. 1,443,149, issued January 23, 1923,

to Philip Schidrowitz). When a vulcanized dispersion is used for coating or impregnation purposes and the dispersion-treated body is then dried to effect coagulationof the rubher, the resulting rubber coagulum exists in a vulcanized condition, so that the step 80 of subsequent heating of the body to efi'ect vulcanization may be avoided.

It is known that the characteristics of a vulcanized dispersion, such as vulcanized latex, may be accurately controlled or varied by blending or Inixin an unvulcanized dispersion therewith. uch a method is particularly advantageous when ultra accelerators are used in eifecting the vulcanization of aqueous rubber dispersions, such as latex, as in such case it is diflicult to control the degree of vulcanization in the resulting product, vulcanization often proceeding beyond the sta e desired. By the simple expedient of b endin an unvulcanized latex with the vulcani'ze latex, it is possible to restore the vulcanized latex to the condition desired for use. Such a method is also particularly advantageous when it is desired to concentrate and/or purify a vulcanized latex by the use of so-called. creaming agents, as

1929. Serial No. 380,454.

we have discovered that whereas little, if any, concentration and/or purification of a vulcanized latex may be effected by the addition of creaming agents thereto, it is possible to effect a marked concentration and /or purification of a mixture of vulcanized and unvul-' canized latices. Various creaming a ents which do not cause coagulation of the ru ber, such as gums or pectin-containing gels, may be used to cause creaming or separationinto a concentrated rubber-containing poi'tion or top layer containing a lower percentage of non-rubber constituents than the original latex, and a lower aqueous layer or serum, whereupon the rubber-containing portion may be recovered. The amount of serum separated from a mixture or blend of natural and vulcanized latex depends upon the amount of natural latex added to the vulcanized latex. The creaming agent, e. g., gum tragacanth, plant sap, or fruit juices, may be added in amount e ual to about 10% by volume of the mixture 0 latices and in the form of about a 2% solution. The rate of creaming may be increased by heating the mixture of latices containing the creaming agent for a short riod of time, a temperature of 70 0., for instance, being maintained for about one-half hour, whereupon the composition may be allowed to stand for about five to twenty-four hours, or possibly a week, to ensure maximum separation of the concentrated rubber-containing portion from the aqueous portion.

The followin table gives the results obtained with different proportions of natural and vulcanized laticesz' From the foregoing table, it" is seen that while considerable creaming is effected in the case of a mixture containing or more of natural latex, comparatively little creaming above that obtained when using vulcanized latex is effected in the case of a mixture containing about 25% natural latex. The steps of mixing unvulcanized latex with natural latex, of adding creaming agents to effect concentrationfand/or purification, and of recovering the creamed portion, may be applied to advantage when it is desired to produce a rubber coagulum of predetermined tensile strength. The tensile strength and stretch characteristics of rubber films made with various mixtures of natural and vulcanized latices are given in the following tables:

. strengt er cent 7 Maw-ml lbs/sq. stretch Natural latex 680 760 757 natural latex 25 /5 vulcanizlcd latex 1025 775 507 natural atex 507g vulcanized latex. 1130 775 25 natural latex 75%? vulcanized latex 1765 800 Vulcanized latex 2810 750 Zecovering the creamed portion of the mixure.

2. A composition of matter comprising a creamed, concentrated mixture of vulcanized latex and more than 25% unvulcanized latex.

3. A composition of matter comprising a purified, concentrated mixture of vulcanized latex and more than 25% unvulcanized latex, which composition has resulted from creaming and separating from a substantial portion of the water component of the unconcentrated mixture and the non-rubber solids present in such water component.

In testimony whereof we have afiixed our slgnatures.

WALLACE B. VAN ARSDEL. ROGER B. HILL.

able of being controlled by the process of the present invention.

vulcanization of the latex may be effected under various conditions, one procedure, for example, being to add about 2% sulphur (based on the solids content of the latex), 0.5% of an accelerator such as'pi eridene penta-methylene dithiocarbamate, a out 5% of zinc oxide, heating to about 75 C. and maintaining this .tem )erature for about two and a half hours. vident-ly vulcanization modifies certain non-rubber constituents of the latex along with the rubber,

as attested by the fact that the property of undergoing creaming possessed by the natural latex is almost lacking in the vulcanized product. If desired, selenium may be used in lieu of sulphur, as a vulcanizing agent, as described in the application of Roger B. Hill, Serial No. 380,468, filed July 23, 1929.

It should be evident to those skilled in the art that the composition ofthe present invention and its mode of preparation may be subject to various changes and modifications without departing from the spirit or scope of invention as defined in the appended claims. i

We claim:

1. A process which comprises mixing vulcanized latex with more than 25% unvulcanlzed latex, adding a creaming agent, and 

